1. Field of the Invention
The present invention relates to a nonreciprocal circuit device and a communication device.
2. Description of the Related Art
A general type of lumped-constant isolator (which is a type of nonreciprocal circuit device) adopted in mobile communication devices, such as portable telephones, serves to transmit signals only in a transmission direction and not in the opposite direction. In recent years, there has been an increasing demand for mobile communication devices with higher reliability and higher performance to satisfy an increasing range of uses. Consequently, there has been a need to improve the reliability and performance of the lumped-constant isolator.
Such a lumped-constant isolator comprises a permanent magnet, a ferrite to which a DC magnetic field is applied by the permanent magnet, a plurality of center electrodes placed on the ferrite, a resin casing for accommodating the ferrite and the center electrodes, upper and lower casings for accommodating the permanent magnet, the ferrite, and the center electrodes, and the like. The upper casing and the lower casing are joined so as to form a magnetic circuit and to also function as a yoke. The upper casing and the lower casing are usually made of a thin plate chiefly containing iron.
Hitherto, studies on the thickness and shape of the thin plate chiefly containing iron have been conducted in order to reduce the size of the isolator and to reduce the leakage of the DC magnetic field applied by the permanent magnet. Furthermore, there have been attempts to reduce the insertion loss of the isolator by plating the surface of the thin plate chiefly containing iron with a material having a low electrical resistance (for example, Ag).
However, no specific limitations have hitherto been placed on the surface roughness of the thin plate chiefly containing iron. This is because the surface roughness varies according to the type of working roll used to produce a rolled steel thin plate. For this reason, there are limitations to the degree to which the insertion loss of the isolator can be reduced.
The present invention provides a nonreciprocal circuit device and a communication device with high reliability and high performance and with reduced insertion loss.
According to an aspect of the present invention, there is provided a nonreciprocal circuit device including a permanent magnet, a ferrite to which a DC magnetic field is applied by the permanent magnet, a plurality of center electrodes placed on the ferrite, and a metal casing for accommodating the permanent magnet, the ferrite, and the center electrodes, wherein the metal casing is made of a material chiefly containing iron, and the average value of the surface roughness of the entirety of the metal casing is less than or equal to 0.9 xcexcm.
Since the average value of the surface roughness of the metal casing is set to be less than or equal to 0.9 xcexcm, the insertion loss of the nonreciprocal circuit device can be reduced, and the reliability and performance of the nonreciprocal circuit device can be improved.
According to another aspect of the present invention, there is provided a communication device having a nonreciprocal circuit device having the above features. This communication device can provide superior high-frequency characteristics.
Further, features and advantages of the present invention will become apparent from the following description of embodiments thereof, with reference to the attached drawings.